Bio-suit zombie

The bio-suit zombie, also known as a hazmat zombie, is a type of zombie introduced in Doom 3: Resurrection of Evil, and occurring again in The Lost Mission. This enemy is rare, occurring in only one level of each expansion. The bio-suit zombies wear a yellow hazmat suit with a flashlight attached to its head, and they behave much like normal zombies, slashing at nearby players. They are first encountered in the toxic waste tunnels in Erebus Sector 5, for which the player has donned an enviro suit and has a limited air supply to traverse the area. When killed, bio-suit zombies drop valuable enviro tanks which top up the player's air supply and help the player stay alive much longer. They appear again in an area of the Mars surface outside the Exis Labs in the Lost Mission, and again, drop air canisters the player can collect.

In the BFG Edition, the enviro suit mechanic was removed from the toxic waste tunnels in Erebus Sector 5; the area still includes bio-suit zombies, but they do not drop enviro tanks.

Development
Screenshots released early in the development of Resurrection of Evil depict the bio-suit zombie as wearing a dark grey hazmat suit that has a red band just below each elbow. This design was originally used for certain NPCs in the base campaign, such as Dr. Michaels and the unknown person who emerges from the Delta 4 teleporter only to be killed immediately by Hell knights. It is unclear if this earlier version of the monster was ever intended to be the final version of the monster, or if the version was merely a stand-in until a later design could be finalized.